Bioavailability (F) Starting concentration (C0), Dose (D) Apparent volume of d (Vd)

PharmacoKinetics Flashcards by O lawformonde

Therapeutic Index :

Formula

LD50/ED50

Bad/Good

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Smaller therapeutic index =

More LETHAL

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MORE efficacy – curve goes

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MORE potency - curve goes

Left

EC50 less

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Quaque die

1 per day

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BID

Bis in die

2 per day

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TID

Ter in die

3x per day

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QID

Quater in die

4x day

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Q#h

Every # hours

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P.r

Rectal

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I.d.

Intradermal

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P.o.

Per os

Oral

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Infusion (continous) for Rx with

Short half life

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Intermittent infusion (short durations at intervals) for Rx w/

Long half life

irritate veins

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Pharmacokinetics

How Rx’s PLASMA CONCENTRATION changes over TIME

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Drug IN

Concepts:

Bioavailability (F)
Starting concentration (C0), Dose (D)
Apparent volume of d (Vd)

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DRUG OUT

Concepts

Vd
1 compartment:
  0-order kinetics
  1-order kinetics
     T1/2, Kel (elimination), Cl (clear)
      Css (steady state), MD/D*
2 compartment

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Steady state needs

Fixed dose at fixed interval

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Affect Co (starting conc)
(Drug in)

Absorption

Distribution

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Affect t1/2

Drug out

Distribution
Metabolism - ELIMINATE
Excretion - ELIMINATE

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Bioavailability (F) =

F = AUC oral/AUC iv
0 = F = 1

Fraction absorbed into plasma

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Dose conversion (oral - iv)

Mg, mg/kg

Di.v. = F x Doral

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Vd =

L\kg

Vd = Di.v./Co

FROM Co = Di.v. / Vd
Conc=Mass/vol

Elimination constant (Kel) =

1/hr

Kel = .7 / t1/2

Clearance (Clp) = | L/hr/kg

Clp = Kel x Vd

Half life (t1/2) = | Hr , min

T1/2 = 0.7 x Vd / Clp

Maintanence Dose (MDi.v.) = mg/hr/kg

MDi.v. = Css x Clp

Interval Dose (Di i.v.) = | Mg/kg

Di i.v. = MDi.v. X interval duration

Loading dose (D*i.v.) = | Mg/kg

D* i.v. = Css x Vd

Zero order =

Single compartment Fixed rate NOT concentration-dependent

1st order =

Single compartment Variable rate Concentration-dependent

2 compartment =

2 first-order decays Two variable rates Concentration-dependent

IV administration , F =

Why need F?

Dose conversion | Iv-oral, MD, D*

Vd | Absolute vs. Relative

Absolute Vd = L Relative Vd = L/kg Absolute = Relative x patient mass (kg)

What if Rx Vd >>> body weight? | or LARGE Rx Vd

Rx residing in tissues | Long t1/2

Plasma Vd

Nonvascular extracellular Vd

16 L

Intracellular Vd =

23 L

Low Vd =

Rx to plasma | Short t1/2

Medium Vd =

Extracell +/- intracell Extracell - out kidney Intracell - metab liver

When can combined compartments be treated as 1?

Distribution rate > elmination rate | Distribution quick

Single compartment phases

Distribution | Elimination (determine t1/2)

Zero order, single compartment example

PEA Phenytoin Ethanol Aspirin

Zero order kinetics - enzyme saturation?

YES

First order -usually concetration BELOW

Km | ...rate dependent on [drug]

First order decay

Concentration drops 50% every half life

Plasma clearance (Clp) assumes

Unit time smaller than t1/2 | ABSOLUTE Vd

Steady state achieved after ___ half lives

90% steady state after __ half lives

3.3

Dose interval vs. continuous infusion

More variation between doses, but same steady state

Loading dose (D*) only used when half life is

LONG

How to ID 2-compartment?

Distribution slower, elimination WAY slower

High Vd = (metab)

Hepatic metab

Low Vd = (metab)

Kidney metab